The present invention generally relates to an air-fuel ratio control system for an internal combustion engine and, more particularly, to the control system for controlling the air-fuel mixing ratio of a combustible air-fuel mixture in dependence on an engine operating condition.
For the purpose of this specification, the terms "leaned operating range" and "enriched operating range" herein used are to be understood as meaning a condition during which the internal combustion engine is operated with the supply of a leaned air-fuel mixture and that during which it is operated with the supply of an enriched air-fuel mixture, respectively.
There is well known an air-fuel ratio control system for controlling the air-fuel ratio of the combustible air-fuel mixture in dependence on an engine operating condition detected in reference to a combination of parameters including, for example, the suction pressure developed inside an intake system of the engine, the quantity (or flow rate) of air being sucked through the intake system and the engine speed. (See, for example, Japanese Laid-open Patent Publication No. 56-115838 laid open to public inspection on Sept. 11, 1981).
For the purpose of minimizing the fuel consumption and the emission of obnoxious exhaust gases, attempts have recently been made to increase the leaned operating range of the engine as large as possible.
However, the increase of the leaned operating range necessarily results in the approximation to the enriched operating range, and will bring about such a problem that even a slight change in engine operation condition may be liable to result in the shift from the leaned operating condition to the enriched operating condition and back to the leaned operating condition with the consequence that an engine torque shock incident to an abrupt change in air-fuel ratio may occur. This will be discussed in detail with particular reference to FIG. 1 of the accompanying drawings which illustrates the change in suction pressure with change in throttle opening while the engine speed is fixed at 1,500 rpm.
As shown in FIG. 1, when and so long as the throttle opening, i.e., the opening of a throttle valve disposed in the inake system of the engine, is smaller than 20 degrees, the suction pressure changes at a great gradient with change in throttle opening, but when and so long as it is equal to or greater than 20 degrees, the change in suction pressure relative to the change in throttle opening becomes extremely small with the result that the responsivity to the change in throttle opening is extremely lowered. (In other words, the suction pressure is brought in a substantially saturated condition when the throttle opening exceeds 20 degrees.) The 20 degrees of the throttle opening referred to above corresponds to the suction pressure (-50 mmHg) attained at the time of maximum foot depression during an EM mode, and if the leaned operating range is increased to this operating range, a slight change in suction pressure which is a factor used to control the air-fuel ratio results in the shift from the leaned operating range to the enriched operating range, and vice versa, accompanied by an abrupt change in air-fuel ratio to such an extent as to result in the torque shock. The above described abrupt change in air-fuel ratio also brings about a change in both the quantity of the air being sucked and the suction pressure, which are factors used to control the air-fuel ratio, resulting in the occurrence of a sort of hunting phenomenon wherein a reciprocating shift between the leaned operating range and the enriched operating range takes place. Once this hunting phenomenon occurs, the drivability will be adversely affected, imposing a limitation on the expansion of the leaned operating range.
Moreover, since the change in quantity of the air being sucked and that in suction pressure are both extremely small during the enriched operating range, the control responsivity tends to be lowered to such an extent as to result in the difficulty in accurate air-fuel control.
For substantially eliminating the above described problems, it may be contemplated to determine a target air-fuel ratio in reference to the throttle opening. However, since during a low load engine operating condition wherein the throttle opening is small, the change in quantity of the air being sucked relative to the throttle opening does not exhibit a linear characteristic, the determination of the target air-fuel ratio is not easy and is not desirable in terms of fuel consumption and drivability.